Originally Posted by JoeD
Tankless heaters take a bunch of power. Find the breakers that control those wires and verify the amperage is correct for your new heater. You may need to change the breaker if it is too high.
Cap one set of wires and use the other set to connect to your heater. Use a meter to determine which set is controlled by which breaker. Yes a box on the wall with a cover is required for the capped wires and to connect to your new heater.
Joe, that points me in the right direction. There are 6 breakers on my fuse panel that are dedicated to the tankless unit, and only 2 conduits going to the unit. I'm guessing those are basically two 220V circuits. Do I just start flipping breakers and use a multi-meter to see which wires have power?